1. Field of the Invention
The present invention relates to an image data correction method and an image data correction apparatus. More specifically, the present invention relates to an image data correction method and an image data correction apparatus for correcting image data read from a stimulable phosphor sheet used repeatedly for radiography. The present invention also relates to a program for causing a computer to execute the image data correction method.
2. Description of the Related Art
A radiography system using a stimulable phosphor (storage phosphor) is well known. A stimulable phosphor stores a part of energy of radiation emitted thereon, and releases the stored energy in the form of light in proportion thereto in response to exposure to a stimulating ray such as visible light. In a radiography system, an image of a subject such as a human body is recorded in a stimulable phosphor sheet, and the sheet is scanned with a stimulating ray such as a laser beam to cause the sheet to emit light. The light is then photoelectrically read to obtain an image signal, and a radiograph generated from the image signal is used for diagnosis. In the radiography system, a residual part of the radiation energy is released from the stimulable phosphor sheet by irradiation of the sheet with an erasing ray after the radiograph is read from the sheet. The stimulable phosphor sheet can then be used again for radiography.
In the radiography system, shading may be observed in the form of uneven density in the radiograph generated in the above manner, due to uneven scanning of the stimulable phosphor sheet with the stimulating ray or due to uneven detection of the light emitted from the sheet. Therefore, a method of measuring an effect of shading (Japanese Unexamined Patent Publication No. 1(1989)-099372) and a method of correcting shading (Japanese Unexamined Patent Publication No. 64(1989)-086759) are known.
Meanwhile, a light emission characteristic of a stimulable phosphor, comprising a stimulable phosphor sheet changes, due to repeated exposure to radiation for radiography. More specifically, a stimulable phosphor is altered by exposure to radiation, and the light emission characteristic (a characteristic in an amount of light emitted upon exposure to a stimulating ray of a predetermined condition from the sheet having been exposed to radiation of a predetermined dose) changes in proportion to an accumulated dose of radiation. Furthermore, in radiography described above, a center part of the sheet generally receives radiation having passed through a target and thus having weaker strength while a peripheral part of the sheet receives radiation not having passed through the target and thus having no change in the strength. Therefore, the dose of radiation increases in the peripheral part than in the center part. In addition, transmissivity varies from part to part in the target. Therefore, even in the center part of the sheet, the radiation exposure varies from point to point.
Consequently, a difference becomes larger in the accumulated dose of radiation between the center part and the peripheral part and among different points in the center part due to repeated radiography of various kinds of subjects. As a result, an amount of light emitted from each point becomes different even in the case where the sheet has received the same amount of radiation and has been exposed to the same amount of stimulating ray. Therefore, uneven density is observed in the image read from the sheet. In other words, uneven density is observed in the radiograph because the light emission characteristic becomes different between areas wherein the accumulated dose of radiation varies. The accumulated dose of radiation refers to a total amount of radiation the sheet has ever received. In the case where the amount of radiation on the sheet changes from point to point as has been described above, the accumulated dose of radiation varies from area to area in the sheet.
Especially, in the case where a stimulable phosphor sheet is used for radiography in a radiation therapy apparatus (such as an apparatus described in Japanese Unexamined Patent Publication No. 9(1997)-239044) for curing by exposing a lesion to radiation of a much larger dose, such as radiation of 1 million times the strength of diagnostic radiography, an amount of radiation on the sheet becomes larger even in a single radiography operation. Therefore, the change in the light emission characteristic becomes apparent in some cases even if the sheet has been used in radiography for a limited number of times.